1. Field of the Invention
The present invention relates to a light-emitting system, more particularly to a light-emitting system having a luminous flux control device.
2. Description of the Related Art
The forward voltage of a light emitting diode (LED) is influenced by the ambient temperature. FIG. 1 shows a plot of luminous flux and forward voltage vs. ambient temperature obtained for the LED when the LED is driven by a continuous wave constant driving current. FIG. 2 shows a plot of luminous flux and forward voltage vs. ambient temperature obtained for the LED when the LED is driven by a non-continuous wave constant driving current. It is evident that a rise in the ambient temperature will cause the forward voltage to fall, such that the luminous flux, which is in a positive relation to the light emitting efficiency or a product of the forward voltage and the operating current, is in a negative relation to the ambient temperature. Hence, application of LED without implementation of luminous flux control may result in instability in the luminous flux of the LED.
Referring to FIG. 3, Taiwanese Patent Application No. 92107029 discloses a conventional luminous flux control circuit 1 for controlling a light emitting power and hence a luminous flux of an LED 15 (e.g., a laser light emitting diode) in an optical pick-up of an optical drive device. The conventional luminous flux control circuit 1 includes a detection module 10, a signal source 11, an integration module 12, and a driving module 13.
The detection module 10 is operable to receive light emitted from the LED 15 and to detect the light emitting power of the LED 15 so as to generate a detection voltage V3 having a magnitude that is in a positive relation to the light emitting power detected by the detection module 10. The light emitting power is defined by the equation of P=VF×I, where P, VF, and I are the light emitting power, a forward voltage, and an operating current of the LED 15, respectively.
The detection module 10 includes a light detector 101 and a front-end amplifier 102. Since a description of the operations of these components may be found in the specification of the aforesaid Taiwanese Application, these components will not be described hereinafter for the sake of brevity.
The signal source 11 is operable to generate a reference voltage V1 that has a magnitude greater than that of the detection voltage V3 and dynamically configurable according to a target light emitting power.
The integration module 12 is connected electrically to the signal source 11 and the detection module 10 for respectively receiving the reference voltage V1 and the detection voltage V3 therefrom, and is operable to output an integration voltage V2 based on an integration of a difference between the reference voltage V1 and the detection voltage V3. When the detection voltage V3 is reduced as a result of a reduction in the light emitting power, the difference between the reference voltage V1 and the detection voltage V3 is increased, causing the integration voltage V2 to increase. On the other hand, when the detection voltage V3 is increased as a result of an increase in the light emitting power, the difference between the reference voltage V1 and the detection voltage V3 is decreased, causing the integration voltage V2 to decrease.
The driving module 13 is interconnected electrically between the integration module 12 and the LED 15, and is operable to generate and provide to the LED 15 the operating current having a magnitude that is in a positive relation to the integration voltage V2 so as to stabilize light emitting power and hence luminous flux of the LED 15. The driving module 13 includes an amplifier 131 having an adjustable gain, and a driving unit 132 electrically connected electrically to the amplifier 131. Since a description of the operations of these components may be found in the specification of the aforesaid Taiwanese Application, these components will not be described hereinafter for the sake of brevity.
When the forward voltage of the LED 15 is decreased as a result of an increase in the ambient temperature, the light emitting power is reduced, the detection voltage V3 generated by the detection module 10 is decreased while the reference voltage V1 remains unchanged, and the difference between the reference voltage V1 and the detection voltage V3 is thus increased such that the integration voltage V2 and hence the operating current are, as a result, increased. This increase in the operating current serves to compensate for the reduction in the forward voltage, thereby stabilizing the light emitting power and hence the luminous flux.
It can be understood from the above that the conventional luminous flux control circuit 1 stabilizes the light emitting power through adjusting the operating current according to variations in the detection voltage V3, which corresponds to variations in light detected by the light detector 101 of the detection module 10. However, since the LED 15 suffers from poor directivity, factors such as distance between and positions of the light detector 101 and the LED 15, ambient light pollution, and sensitivity of the light detector 101 may cause errors in stabilization of the light emitting power, such that the conventional luminous flux control circuit 1 may not be able to effectively stabilize the light emitting power and hence the luminous flux of the LED 15 in response to variations in the ambient temperature.